Light-emitting element, light-emitting device, and display device

ABSTRACT

A light-emitting element with which a reduction in power consumption and an improvement in productivity of a display device can be achieved is provided. A technique of manufacturing a display device with high productivity is provided. The light-emitting element includes an electrode having a reflective property, and a first light-emitting layer, a charge generation layer, a second light-emitting layer, and an electrode having a light-transmitting property stacked in this order over the electrode having a reflective property. The optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of the peak wavelength of the emission spectrum of the first light-emitting layer. The optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of the peak wavelength of the emission spectrum of the second light-emitting layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a light-emitting element exhibiting electroluminescence, a display device including the light-emitting element, and manufacturing methods of the light-emitting element and the display device.

2. Description of the Related Art

In recent years, an electroluminescent (also referred to as EL) display device has attracted attention as a display device with reduced thickness and weight (i.e., so-called flat panel display).

Light-emitting elements using light-emitting materials emitting light of different colors are provided as light-emitting elements used in pixels in an EL display device, so that full-color display can be performed.

For such an EL display device, a method is used in which selective deposition of light-emitting materials in a minute pattern is performed for each pixel by an evaporation method using a metal mask.

However, a shape defect or emission defect might be caused to a light-emitting element due to contact of a metal mask, and ways to prevent the defects have been explored (e.g., Patent Document 1). Patent Document 1 discloses a structure in which a spacer for supporting a metal mask is provided over a pixel electrode so that the metal mask and the pixel electrode are not in contact with each other at the time of evaporation.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.     2006-126817

SUMMARY OF THE INVENTION

A method in which selective deposition of light-emitting materials is performed for each pixel has a complicated process; thus, yield or productivity is difficult to increase.

An object of one embodiment of the present invention is to provide a light-emitting element with which a reduction in power consumption and an increase in productivity of a display device can be achieved.

An object of one embodiment of the present invention is to provide a technique for manufacturing a display device with high productivity.

An object of one embodiment of the present invention is to provide a high-definition display device with high color purity.

One embodiment of the present invention is a light-emitting element including an electrode having a reflective property, and a first light-emitting layer, a charge generation layer, a second light-emitting layer, and an electrode having a light-transmitting property which are stacked in this order over the electrode having a reflective property. The optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of the peak wavelength of the emission spectrum of the first light-emitting layer. The optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of the peak wavelength of the emission spectrum of the second light-emitting layer. Note that in this specification, a peak wavelength of an emission spectrum refers to a wavelength of the highest emission intensity peak of a plurality of peaks in an emission spectrum.

In the above structure, the first light-emitting layer and the second light-emitting layer emit light of different colors.

One embodiment of the present invention is a light-emitting element including an electrode having a reflective property, and a first light-emitting layer, a first charge generation layer, a second light-emitting layer, a second charge generation layer, a third light-emitting layer, and an electrode having a light-transmitting property which are stacked in this order over the electrode having a reflective property. The optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of the peak wavelength of the emission spectrum of the first light-emitting layer. The optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of the peak wavelength of the emission spectrum of the second light-emitting layer. The optical path length between the electrode having a reflective property and the third light-emitting layer is five-quarters of the peak wavelength of the emission spectrum of the third light-emitting layer.

In the above structure, the first light-emitting layer, the second light-emitting layer, and the third light-emitting layer emit light of different colors.

One embodiment of the present invention is a display device including a plurality of light-emitting elements and a plurality of color filter layers provided over the plurality of light-emitting elements. Each of the plurality of light-emitting elements includes an electrode having a reflective property, and a first light-emitting layer, a charge generation layer, a second light-emitting layer, and an electrode having a light-transmitting property which are stacked in this order over the electrode having a reflective property. The optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of the peak wavelength of the emission spectrum of the first light-emitting layer. The optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of the peak wavelength of the emission spectrum of the second light-emitting layer. The plurality of color filter layers include at least a color filter layer which transmits light with a peak wavelength of the emission spectrum of the first light-emitting layer and a color filter layer which transmits light with a peak wavelength of the emission spectrum of the second light-emitting layer.

One embodiment of the present invention is a display device including a plurality of light-emitting elements and a plurality of color filter layers provided over the plurality of light-emitting elements. Each of the plurality of light-emitting elements includes an electrode having a reflective property, and a first light-emitting layer, a first charge generation layer, a second light-emitting layer, a second charge generation layer, a third light-emitting layer, and an electrode having a light-transmitting property which are stacked in this order over the electrode having a reflective property. The optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of the peak wavelength of the emission spectrum of the first light-emitting layer. The optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of the peak wavelength of the emission spectrum of the second light-emitting layer. The optical path length between the electrode having a reflective property and the third light-emitting layer is five-quarters of the peak wavelength of the emission spectrum of the third light-emitting layer. The plurality of color filter layers include at least a color filter layer which transmits light with a peak wavelength of the emission spectrum of the first light-emitting layer, a color filter layer which transmits light with a peak wavelength of the emission spectrum of the second light-emitting layer, and a color filter layer which transmits light with a peak wavelength of the emission spectrum of the third light-emitting layer.

According to one embodiment of the present invention, a display device can be manufactured with high productivity.

Further, according to one embodiment of the present invention, a high-definition display device with high color purity can be provided.

Further, according to one embodiment of the present invention, a display device with low power consumption can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views each illustrating a display device.

FIG. 2 is a view illustrating a display device.

FIGS. 3A to 3C are views illustrating display devices.

FIGS. 4A and 4B are views illustrating a display device.

FIGS. 5A to 5F are views each illustrating an example of application of a display device.

FIG. 6 is a view illustrating a light-emitting element of Example.

FIG. 7 is a graph showing an emission spectrum of the light-emitting element of Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the following description, and it will be easily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments below. In the structures to be given below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and explanation thereof will not be repeated.

Embodiment 1

In this embodiment, one embodiment of an EL display device will be described with reference to FIGS. 1A and 1B, FIG. 2, and FIGS. 3A to 3C.

FIG. 1A illustrates a structure of a light-emitting element according to this embodiment. The light-emitting element illustrated in FIG. 1A includes an electrode 102 having a reflective property, and a first EL layer 106, a charge generation layer 107, a second EL layer 110, and an electrode 112 having a light-transmitting property which are stacked in this order over the electrode 102 having a reflective property. Light emitted from the light-emitting element is emitted from the electrode 112 side.

In FIG. 1A, the first EL layer 106 includes at least a first light-emitting layer 120. The second EL layer 110 includes at least a second light-emitting layer 122. Note that each of the first EL layer 106 and the second EL layer 110 can have a stacked-layer structure including functional layers such as a hole-injection layer, a hole-transport layer, an electron-transport layer, and an electron-injection layer, in addition to the light-emitting layer.

Light emitted from the first light-emitting layer 120 in each of the light-emitting elements illustrated in FIGS. 1A and 1B is extracted outside the light-emitting element through the electrode 112 having a light-transmitting property. Here, components of light entering the electrode 112 having a light-transmitting property include light entering the electrode 112 having a light-transmitting property directly from the first light-emitting layer 120 (the light is also referred to as first entering light) and light which is emitted from the first light-emitting layer 120 and is reflected back by the electrode 102 having a reflective property (the light is also referred to as first reflected light). The first entering light and the first reflected light interfere with each other. Thus, the optical path length between the electrode 102 having a reflective property and the first light-emitting layer 120 is adjusted, whereby the phase of the first entering light and the phase of the first reflected light can be aligned with each other.

In this embodiment, the optical path length between the electrode 102 having a reflective property and the first light-emitting layer 120 is set to one-quarter of the peak wavelength of the emission spectrum of the first light-emitting layer. This allows the phases of the first entering light and the first reflected light to be aligned and emission from the first light-emitting layer 120 to be increased. Thus, the luminance of the light-emitting element according to this embodiment is higher than the luminance of a light-emitting element in which the optical path length is not adjusted, in the case where the same current is applied to these light-emitting elements.

In a similar manner, light emitted from the second light-emitting layer 122 is extracted outside the light-emitting element through the electrode 112 having a light-transmitting property. Components of light entering the electrode 112 having a light-transmitting property include light entering the electrode 112 having a light-transmitting property directly from the second light-emitting layer 122 (the light is also referred to as second entering light) and light which is emitted from the second light-emitting layer 122 and is reflected back by the electrode 102 having a reflective property (the light is also referred to as second reflected light). The second entering light and the second reflected light interfere with each other. Thus, the optical path length between the electrode 102 having a reflective property and the second light-emitting layer 122 is adjusted, whereby the phase of the second entering light and the phase of the second reflected light can be aligned with each other.

In this embodiment, the optical path length between the electrode 102 having a reflective property and the second light-emitting layer 122 is set to three-quarters of the peak wavelength of the emission spectrum of the second light-emitting layer. This allows the phases of the second entering light and the second reflected light to be aligned and emission from the second light-emitting layer 122 to be increased. Thus, the luminance of the light-emitting element according to this embodiment is higher than the luminance of a light-emitting element in which the optical path length is not adjusted, in the case where the same current is applied to these light-emitting elements.

More strictly, the optical path length between the electrode 102 having a reflective property and the first light-emitting layer 120 can also be referred to as the optical path length between the electrode 102 having a reflective property and a light-emitting region in the first light-emitting layer 120. Note that it is difficult to strictly determine the position of the light-emitting region in the light-emitting layer and that the effects described above can be sufficiently obtained by assuming any position in the light-emitting layer as the position of the light-emitting region. In other words, the optical path length between the electrode 102 having a reflective property and the first light-emitting layer 120 can be referred to as the optical path length between a surface of the electrode 102 having a reflective property and a lower surface of the first light-emitting layer 120 or more and the optical path length between the surface of the electrode 102 having a reflective property and an upper surface of the first light-emitting layer 120 or less. The same can be applied to the optical path length between the electrode having a reflective property 102 and the second light-emitting layer 122, and the optical path length between the electrode 102 having a reflective property and the third light-emitting layer 124 which will be described later.

Note that it is preferable that the first light-emitting layer 120 and the second light-emitting layer 122 emit light of different colors.

The structure of the light-emitting element illustrated in FIG. 1A will be described below along with specific materials. Note that an element structure, a manufacturing method, and the like described here are just examples, and other known structures, materials, and manufacturing methods can be applied without departing from the purpose of this embodiment.

The electrode 102 having a reflective property is provided opposite to the side where light is extracted and is formed using a reflective material. As the reflective material, a metal material such as aluminum, gold, platinum, silver, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium can be used. In addition, any of the following can be used: alloys containing aluminum (aluminum alloys) such as an alloy of aluminum and titanium, an alloy of aluminum and nickel, and an alloy of aluminum and neodymium; and an alloy containing silver such as an alloy of silver and copper. An alloy of silver and copper is preferable because of its high heat resistance. Further, a metal film or a metal oxide film is stacked on an aluminum alloy film, whereby oxidation of the aluminum alloy film can be prevented. As examples of a material of the metal film or the metal oxide film, titanium, titanium oxide, and the like are given. The above materials are preferable because they are present in large amounts in the Earth's crust and inexpensive to achieve a reduction in manufacturing cost of a light-emitting element.

In this embodiment, the case where the electrode 102 having a reflective property is used as an anode of the light-emitting element is described as an example. However, one embodiment of the present invention is not limited thereto.

A conductive layer having a light-transmitting property may be provided on and in contact with the electrode 102 having a reflective property so that the optical path length between the electrode 102 having a reflective property and the first light-emitting layer 120. The conductive layer having a light-transmitting property is formed of a single layer or stacked layers using a material having a property of transmitting visible light. As the light-transmitting material, for example, indium oxide, indium tin oxide, indium oxide-zinc oxide, zinc oxide, zinc oxide to which gallium is added, graphene, or the like can be used.

The conductive layer having a light-transmitting property can be formed using a conductive composition containing a conductive high molecule (also referred to as conductive polymer). As the conductive high molecule, a so-called π-electron conjugated conductive polymer can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene (PEDOT) or a derivative thereof, a copolymer of two or more of aniline, pyrrole, and thiophene or a derivative thereof, and the like can be given.

The first EL layer 106 may include at least the first light-emitting layer 120. In addition, the first EL layer 106 can have a stacked-layer structure in which a layer containing a substance having a high hole-transport property, a layer containing a substance having a high electron-transport property, a layer containing a substance having a high hole-injection property, a layer containing a substance having a high electron-injection property, a layer containing a bipolar substance (a substance having a high hole-transport and electron-transport properties), and the like are combined as appropriate. For example, the first EL layer 106 can have a stacked-layer structure including a hole-injection layer, a hole-transport layer, the first light-emitting layer 120, an electron-transport layer, and an electron-injection layer. Needless to say, in the case where the electrode 102 having a reflective property is used as a cathode, a stacked-layer structure in which an electron-injection layer, an electron-transport layer, the first light-emitting layer 120, a hole-transport layer, and a hole-injection layer are stacked in this order from the cathode side may be employed.

The hole-injection layer is a layer containing a substance having a high hole-injection property. As the substance having a high hole-injection property, for example, metal oxides such as molybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungsten oxide, and manganese oxide can be used. A phthalocyanine-based compound such as phthalocyanine (abbreviation: H₂Pc), or copper(II) phthalocyanine (abbreviation: CuPc) can also be used.

Any of the following aromatic amine compounds which are low molecular organic compounds can also be used: 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), 4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (abbreviation: DPA3B), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), and 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1).

Any of the high molecular compounds (e.g., oligomers, dendrimers, or polymers) can also be used. Examples of the high molecular compound include poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N′-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), and poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation: Poly-TPD). A high molecular compound to which acid is added, such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS) or polyaniline/poly(styrenesulfonic acid) (PAni/PSS), can also be used.

In particular, it is preferable to use a composite material in which an acceptor substance is mixed with an organic compound having a high hole-transport property for the hole-injection layer. With the use of the composite material in which an acceptor substance is mixed with a substance having a high hole-transport property, excellent hole injection from the anode can be obtained, which results in a reduction in the driving voltage of the light-emitting element. Such a composite material can be formed by co-evaporation of a substance having a high hole-transport property and a substance having an acceptor property. When the hole-injection layer is formed using the composite material, holes are easily injected into the first EL layer 106 from the anode.

As the organic compound for the composite material, a variety of compounds such as an aromatic amine compound, carbazole derivatives, aromatic hydrocarbon, and a high molecular compound (such as oligomer, dendrimer, or polymer) can be used. The organic compound used for the composite material is preferably an organic compound having a high hole-transport property. Specifically, a substance having a hole mobility of 10⁻⁶ cm²/Vs or higher is preferably used. Note that any other substances may also be used as long as the hole-transport property thereof is higher than the electron-transport property thereof. Specific examples of the organic compound that can be used for the composite material will be given below.

As the organic compound that can be used for the composite material, any of the following can be used: aromatic amine compounds such as TDATA, MTDATA, DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), and 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), and carbazole derivatives, such as 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA), and 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.

Any of the following aromatic hydrocarbon compounds can also be used: 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene, 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene (abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA), 9,10-bis[2-(1-naphthyl)phenyl)-2-tert-butylanthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene, and 2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene.

Any of the following aromatic hydrocarbon compounds can also be used: 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl, 10,10′-diphenyl-9,9′-bianthryl, 10,10′-bis(2-phenylphenyl)-9,9′-bianthryl, 10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene, tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi), or 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA).

Examples of the electron acceptor include organic compounds such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F₄-TCNQ) and chloranil; and transition metal oxides. Other examples include oxides of metals belonging to Groups 4 to 8 in the periodic table. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because of their high electron-accepting properties. Among these, molybdenum oxide is particularly preferable because it is stable in the air, has a low hygroscopic property, and is easily handled.

Note that the hole injection layer may be formed using a composite material of the high molecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD, and the electron acceptor.

Note that in the case where a layer containing the above composite material is provided in the first EL layer 106, the optical path length between the electrode 102 having a reflective property and the first light-emitting layer 120 (the optical path length of the first reflected light) may be adjusted by adjusting the thickness of the layer containing the above composite material. In that case, the conductive layer having a light-transmitting property is not necessarily provided.

The hole-transport layer is a layer containing a substance having a high hole-transport property. As the substance having a high hole-transport property, any of the following aromatic amine compounds can be used, for example: NPB, TPD, BPAFLP, 4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: DFLDPBi), and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB). The substances given here are mainly ones having a hole mobility of 10⁻⁶ cm²/Vs or higher. However, any other substances may also be used as long as the hole-transport property thereof is higher than the electron-transport property thereof. The layer containing a substance having a high hole-transport property is not limited to a single layer, and may be a stack of two or more layers containing any of the above substances.

A carbazole derivative such as CBP, CzPA, or PCzPA or an anthracene derivative such as t-BuDNA, DNA, or DPAnth may be used for the hole-transport layer.

Alternatively, a high molecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD can be used for the hole-transport layer.

The first light-emitting layer 120 is a layer containing a light-emitting organic compound. As the light-emitting organic compound, for example, a fluorescent compound which emits fluorescence or a phosphorescent compound which emits phosphorescence can be used.

As the fluorescent compound that can be used for the first light-emitting layer 120, a material for blue light emission, a material for green light emission, a material for yellow light emission, and a material for red light emission are given. Examples of the material for blue light emission include N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine (abbreviation: YGA2S), 4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (abbreviation: YGAPA), and 4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBAPA). Example of the material for green light emission include N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCABPhA), N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPABPhA), N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine (abbreviation: 2YGABPhA), and N,N,9-triphenylanthracen-9-amine (abbreviation: DPhAPhA). Examples of the material for yellow light emission include rubrene and 5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT). Examples of the material for red light emission include N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation: p-mPhTD) and 7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (abbreviation: p-mPhAFD).

As the phosphorescent compound that can be used for the first light-emitting layer 120, a material for blue light emission, a material for green light emission, a material for yellow light emission, a material for orange light emission, a material for red light emission are given. Examples of the material for blue light emission include bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6), bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)picolinate (abbreviation: FIrpic), bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)), and bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIr(acac)). Examples of the material for green light emission include tris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃), bis(2-phenylpyridinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(ppy)₂(acac)), bis(1,2-diphenyl-1H-benzimidazolato)iridium(III)acetylacetonate (abbreviation: Ir(pbi)₂(acac)), bis(benzo[h]quinolinato)iridium (III)acetylacetonate (abbreviation: Ir(bzq)₂(acac)), and tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃).

Examples of the material for yellow light emission include bis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(dpo)₂(acac)), bis[2-(4′-(perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate (abbreviation: Ir(p-PF-ph)₂(acac)), bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(bt)₂(acac)), (acetylacetonato)bis[2,3-bis(4-fluorophenyl)-5-methylpyrazinato]iridium(III) (abbreviation: Ir(Fdppr-Me)₂(acac)), and (acetylacetonato)bis{2-(4-methoxyphenyl)-3,5-dimethylpyrazinato}iridium(III) (abbreviation: Ir(dmmoppr)₂(acac)). Examples of the material for orange light emission include tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(pq)₃), bis(2-phenylquinolinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(pq)₂(acac)), (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-Me)₂(acac)), and (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-iPr)₂(acac)). Examples of the material for red light emission include organometallic complexes such as bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′))iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac)), bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(piq)₂(acac)), (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq)₂(acac)), (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr)₂(acac)), (dipivaloylmethanato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr)₂(dpm)), and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrinplatinum(II) (abbreviation: PtOEP). In addition, rare earth metal complexes, such as tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation: Tb(acac)₃(Phen)), tris(1,3-diphenyl-1,3-propanedionato) (monophenanthroline)europium(III) (abbreviation: Eu(DBM)₃(Phen)), and tris[1-(2-thenyl)-3,3,3-trifluoroacetonato] (monophenanthroline)europium(III) (abbreviation: Eu(TTA)₃(Phen)), exhibit light emission from rare earth metal ions (electron transition between different multiplicities), and thus can be used as phosphorescent compounds.

Note that the first light-emitting layer 120 may have a structure in which any of the above light-emitting organic compounds (a guest material) is dispersed in another substance (a host material). As a host material, various kinds of materials can be used, and it is preferable to use a substance which has a lowest unoccupied molecular orbital level (LUMO level) higher than the light-emitting substance and has a highest occupied molecular orbital level (HOMO level) lower than that of the light-emitting substance.

As the host material, specifically, any of the following can be used: metal complexes such as tris(8-quinolinolato)aluminum(III) (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq), bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), and bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); heterocyclic compounds such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), and bathocuproine (BCP); condensed aromatic compounds such as 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 9,9′-bianthryl (abbreviation: BANT), 9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS), 9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2), 3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3), 9,10-diphenylanthracene (abbreviation: DPAnth), and 6,12-dimethoxy-5,11-diphenylchrysene; and aromatic amine compounds such as N,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: CzA1A), 4-(10-phenyl-9-anthryl)triphenylamine (abbreviation: DPhPA), N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: PCAPA), N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine (abbreviation: PCAPBA), N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), NPB, TPD, DFLDPBi, and BSPB.

Plural kinds of materials can be used as the host material. For example, in order to suppress crystallization, a substance such as rubrene which suppresses crystallization, may be further added. In addition, NPB, Alq, or the like may be further added in order to efficiently transfer energy to the guest material.

When the structure in which a guest material is dispersed in a host material is employed, crystallization of the first light-emitting layer 120 can be suppressed. In addition, concentration quenching due to high concentration of the guest material can be suppressed.

A high molecular compound can be used for the first light-emitting layer 120. As specific examples of the high molecular compound, a material for blue light emission, a material for green light emission, and a material for orange to red light emission are given. Examples of the material for blue light emission include poly(9,9-dioctylfluorene-2,7-diyl) (abbreviation: PFO), poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,5-dimethoxybenzene-1,4-diyl)] (abbreviation: PF-DMOP), and poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-(p-butylphenyl)-1,4-diaminobenzene]} (abbreviation: TAB-PFH). Examples of the material for green light emission include polyp-phenylenevinylene) (abbreviation: PPV), poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazole-4,7-diyl)] (abbreviation: PFBT), and poly[(9,9-dioctyl-2,7-divinylenfluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)]. Examples of the material for orange to red light emission include poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene] (abbreviation: MEH-PPV), poly(3-butylthiophene-2,5-diyl) (abbreviation: R4-PAT), poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenyl amino)-1,4-phenylene]}, and poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]} (abbreviation: CN-PPV-DPD).

Note that the first EL layer 106 may have a structure including two or more light-emitting layers.

The electron-transport layer is a layer containing a substance having a high electron-transport property. As the substance having a high electron-transport property, any of the following can be used, for example: metal complexes having a quinoline skeleton or a benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂), and bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation: BAlq). A metal complex or the like including an oxazole-based or thiazole-based ligand, such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂) can also be used. Other than the metal complexes, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), or the like can also be used. The substances given here are mainly ones having an electron mobility of 10⁻⁶ cm²/Vs or higher. The electron-transport layer is not limited to a single layer and may be a stack of two or more layers containing any of the above substances.

The electron-injection layer is a layer containing a substance having a high electron-injection property. For the electron-injection layer, an alkali metal, an alkaline-earth metal, or a compound thereof, such as lithium, cesium, calcium, lithium fluoride, cesium fluoride, calcium fluoride, or lithium oxide, can be used. Alternatively, a rare earth metal compound such as erbium fluoride can be used. Further alternatively, any of the above substances for forming the electron-transport layer can be used.

Charges are generated in the charge generation layer 107 by applying voltage to the light-emitting element. The charge generation layer 107 has functions of injecting holes into the EL layer on the cathode side and injecting electrons into the EL layer on the anode side.

The charge generation layer 107 can be formed using the above composite material. The charge generation layer 107 may have a stacked-layer structure including a layer containing the composite material and a layer containing another material. In that case, as the layer containing another material, a layer containing an electron donating substance and a substance with high electron-transport properties, a layer formed of a transparent conductive film, or the like can be used. As for a light-emitting element having such a structure, problems such as energy transfer and quenching occur with difficulty, and a light-emitting element which has both high light emission efficiency and long lifetime can be easily obtained due to expansion in the choice of materials. Moreover, a light-emitting element which provides phosphorescence from one EL layer and fluorescence from another EL layer can be easily obtained.

Note that the optical path length between the electrode 102 having a reflective property and the second light-emitting layer 122 (the optical path length of the second reflected light) can be adjusted by adjusting the thickness of the charge generation layer 107.

When the charge generation layer is provided between the stacked EL layers, the element can have high luminance and long lifetime while the current density is kept low. In addition, a voltage drop due to the resistance of the electrode material can be reduced, whereby uniform light emission in a large area is possible.

The second EL layer 110 may include at least the second light-emitting layer 122. In addition, the second EL layer 110 can have a stacked-layer structure in which a layer containing a substance having a high hole-transport property, a layer containing a substance having a high electron-transport property, a layer containing a substance having a high hole-injection property, a layer containing a substance having a high electron-injection property, a layer containing a bipolar substance (a substance having a high hole-transport and electron-transport properties), and the like are combined as appropriate. The second EL layer 110 may have a structure similar to that of the first EL layer 106 or may have a stacked-layer structure different from that of the first EL layer 106. For example, the second EL layer 110 can have a stacked-layer structure including a hole-injection layer, a hole-transport layer, the second light-emitting layer 122, an electron-transport layer, an electron-injection buffer layer, an electron-relay layer, and a composite material layer in contact with the electrode 112 having a light-transmitting property. Note that the second EL layer 110 may have a structure including two or more light-emitting layers.

The composite material layer in contact with the electrode 112 having a light-transmitting property is preferably provided, in which case damage caused to the second EL layer 110 particularly when the electrode 112 having a light-transmitting property is formed by a sputtering method can be reduced. The composite material layer can be formed using the above-described composite material in which an acceptor substance is mixed with an organic compound having a high hole-transport property.

Further, by providing the electron-injection buffer layer, an injection barrier between the composite material layer and the electron-transport layer can be reduced; thus, electrons generated in the composite material layer can be easily injected into the electron-transport layer.

A substance having a high electron-injection property can be used for the electron-injection buffer layer: for example, an alkali metal, an alkaline earth metal, a rare earth metal, a compound of the above metal (e.g., an alkali metal compound (including an oxide such as lithium oxide, a halide, or carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (including an oxide, a halide, or carbonate), or a rare earth metal compound (including an oxide, a halide, or carbonate).

In the case where the electron-injection buffer layer contains a substance having a high electron-transport property and a donor substance, the donor substance is preferably added so that the mass ratio of the donor substance to the substance having a high electron-transport property ranges from 0.001:1 to 0.1:1. Note that as the donor substance, an organic compound such as tetrathianaphthacene (abbreviation: TTN), nickelocene, or decamethylnickelocene can be used as well as an alkali metal, an alkaline earth metal, a rare earth metal, a compound of the above metal (e.g., an alkali metal compound (e.g., an oxide such as lithium oxide, a halide, and a carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (e.g., an oxide, a halide, and a carbonate), and a rare earth metal compound (e.g., an oxide, a halide, and a carbonate). Note that as the substance having a high electron-transport property, a material similar to the material for the electron transport layer described above can be used.

Furthermore, it is preferable that the electron-relay layer be formed between the electron-injection buffer layer and the composite material layer. The electron-relay layer is not necessarily provided; however, by providing the electron-relay layer having a high electron-transport property, electrons can be rapidly transported to the electron-injection buffer layer.

The structure in which the electron-relay layer is sandwiched between the composite material layer and the electron-injection buffer layer is a structure in which the acceptor substance contained in the composite material layer and the donor substance contained in the electron-injection buffer layer are less likely to interact with each other; thus, their functions hardly interfere with each other. Therefore, an increase in the driving voltage can be prevented.

The electron-relay layer contains a substance having a high electron-transport property and is formed so that the LUMO level of the substance having a high electron-transport property is located between the LUMO level of the acceptor substance contained in the composite material layer and the LUMO level of the substance having a high electron-transport property contained in the electron-transport layer. In the case where the electron-relay layer contains a donor substance, the donor level of the donor substance is controlled so as to be located between the LUMO level of the acceptor substance in the composite material layer and the LUMO level of the substance having a high electron-transport property contained in the electron-transport layer. As a specific value of the energy level, the LUMO level of the substance having a high electron-transport property contained in the electron-relay layer is preferably greater than or equal to −5.0 eV, more preferably greater than or equal to −5.0 eV and less than or equal to −3.0 eV.

It is preferable to use a phthalocyanine-based material or a metal complex having a metal-oxygen bond and an aromatic ligand as the substance having a high electron-transport property contained in the electron-relay layer.

As the phthalocyanine-based material contained in the electron relay layer, specifically, any of the following is preferably used: CuPc, a phthalocyanine tin(II) complex (SnPc), a phthalocyanine zinc complex (ZnPc), cobalt(II) phthalocyanine, β-form (CoPc), phthalocyanine iron (FePc), and vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (PhO-VOPc).

As the metal complex having a metal-oxygen bond and an aromatic ligand, which is contained in the electron-relay layer, a metal complex having a metal-oxygen double bond is preferably used. The metal-oxygen double bond has an acceptor property (a property of easily accepting electrons); thus, electrons can be transferred (donated and accepted) more easily. Further, the metal complex which has a metal-oxygen double bond is considered stable. Thus, the use of the metal complex having the metal-oxygen double bond makes it possible to drive the light-emitting element at low voltage more stably.

A phthalocyanine-based material is preferable as a metal complex having a metal-oxygen bond and an aromatic ligand. Specifically, any of vanadyl phthalocyanine (VOPc), a phthalocyanine tin(IV) oxide complex (SnOPc), and a phthalocyanine titanium oxide complex (TiOPc) is preferable because a metal-oxygen double bond is more likely to act on another molecular in teens of a molecular structure and an acceptor property is high.

Note that a phthalocyanine-based material having a phenoxy group is preferable as the phthalocyanine-based materials described above. Specifically, a phthalocyanine derivative having a phenoxy group, such as PhO-VOPc, is preferable. A phthalocyanine derivative having a phenoxy group is soluble in a solvent. Thus, a phthalocyanine derivative has an advantage of being easily handled during formation of the light-emitting element and an advantage of facilitating maintenance of an apparatus used for forming a film.

The electron-relay layer may further contain a donor substance. As the donor substance, any of the following can be used: an organic compound such as tetrathianaphthacene (abbreviation: TTN), nickelocene, and decamethylnickelocene, in addition to an alkali metal, an alkaline earth metal, a rare earth metal, and a compound of the above metals (e.g., an alkali metal compound (e.g., an oxide such as lithium oxide, a halide, and a carbonate such as lithium carbonate or cesium carbonate), an alkaline earth metal compound (e.g., an oxide, a halide, and a carbonate), and a rare earth metal compound (e.g., an oxide, a halide, and a carbonate)). When such a donor substance is contained in the electron-relay layer, electrons can be transferred easily and the light-emitting element can be driven at lower voltage.

In the case where a donor substance is contained in the electron-relay layer, in addition to the materials described above as the substance having a high electron-transport property, a substance having a LUMO level greater than the acceptor level of the acceptor substance contained in the composite material layer can be used. Specifically, it is preferable to use a substance having a LUMO level of greater than or equal to −5.0 eV, preferably greater than or equal to −5.0 eV and less than or equal to −3.0 eV. As examples of such a substance, a perylene derivative and a nitrogen-containing condensed aromatic compound are given. Note that a nitrogen-containing condensed aromatic compound is preferably used for the electron-relay layer because of its stability.

Specific examples of the perylene derivative include 3,4,9,10-perylenetetracarboxylicdianhydride (abbreviation: PTCDA), 3,4,9,10-perylenetettracarboxylic-bis-benzimidazole (abbreviation: PTCBI), N,N′-dioctyl-3,4,9,10-perylenetetracarboxylic diimide (abbreviation: PTCDI-C8H), and N,N′-dihexyl-3,4,9,10-perylenetetracarboxylic diimide (Hex PTC).

Specific examples of the nitrogen-containing condensed aromatic compound include pirazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (PPDN), 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT(CN)₆), 2,3-diphenylpyrido[2,3-b]pyrazine (2PYPR), and 2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (F2PYPR).

Other than the above, 7,7,8,8-tetracyanoquinodimethane (abbreviation: TCNQ), 1,4,5,8-naphthalenetetracarboxylicdianhydride (abbreviation: NTCDA), perfluoropentacene, copper hexadecafluoro phthalocyanine (abbreviation: F₁₆CuPc), N,N′-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-1,4,5,8-naphthalenetetracarboxylic diimide (abbreviation: NTCDI-C8F), 3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophene (abbreviation: DCMT), a methanofullerene (e.g., [6,6]-phenyl C₆₁ butyric acid methyl ester), or the like can be used.

Note that in the case where a donor substance is contained in the electron-relay layer, the electron-relay layer may be formed by a method such as co-evaporation of the substance having a high electron-transport property and the donor substance.

Each of the hole-injection layer, the hole-transport layer, the second light-emitting layer 122, and the electron-transport layer may be formed using any of the materials given above. However, a material different from the light-emitting material of the first light-emitting layer 120 is preferably used as a light-emitting material for the second light-emitting layer 122.

The electrode 112 having a light-transmitting property is provided on the side where light is extracted, and thus is formed using a light-transmitting material. As the light-transmitting material, indium oxide, indium tin oxide, indium oxide-zinc oxide, zinc oxide, zinc oxide to which gallium is added, graphene, or the like can be used.

As the electrode 112 having a light-transmitting property, a metal material such as gold, platinum, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, or titanium can be used. A nitride of the metal material (e.g., titanium nitride) or the like may be used. In the case of using the metal material (or the nitride thereof), the electrode 112 having a light-transmitting property may be thinned so as to have a light-transmitting property.

The half width of an emission spectrum of each light-emitting layer in the above-described light-emitting element illustrated in FIG. 1A can be reduced, so that the light-emitting element can have a sharp emission spectrum. In addition, the light-emitting element can have high efficiency.

FIG. 1B illustrates a structure of a light-emitting element which is different from the structure of the light-emitting element illustrated in FIG. 1A. The structure illustrated in FIG. 1B has many parts in common with the structure of the light-emitting element illustrated in FIG. 1A. Therefore, in the following description, the same parts will not be described.

The light-emitting element illustrated in FIG. 1B includes the electrode 102 having a reflective property, and the first EL layer 106, a first charge generation layer 108, the second EL layer 110, a second charge generation layer 114, a third EL layer 118, and the electrode 112 having a light-transmitting property which are stacked in this order over the electrode 102 having a reflective property. The light-emitting element illustrated in FIG. 1B is different from the light-emitting element illustrated in FIG. 1A in that the second charge generation layer 114 and the third EL layer 118 are provided between the second EL layer 110 and the electrode 112 having a light-transmitting property. The other parts of the structure are the same as those of the light-emitting element illustrated in FIG. 1A.

In the light-emitting element illustrated in FIG. 1B, the third EL layer 118 includes at least the third light-emitting layer 124. Light emitted from the third light-emitting layer 124 is extracted outside the light-emitting element through the electrode 112 having a light-transmitting property. Components of light entering the electrode 112 having a light-transmitting property include light entering the electrode 112 having a light-transmitting property directly from the third light-emitting layer 124 (the light is also referred to as third entering light) and light which is emitted from the third light-emitting layer 124 and is reflected back by the electrode 102 having a reflective property (the light is also referred to as third reflected light). The third entering light and the third reflected light interfere with each other. Thus, the optical path length between the electrode 102 having a reflective property and the third light-emitting layer 124 is adjusted, whereby the phase of the third entering light and the phase of the third reflected light can be aligned with each other.

In FIG. 1B, the optical path length between the electrode 102 having a reflective property and the third light-emitting layer 124 is five-quarters of the peak wavelength of the emission spectrum of the third light-emitting layer. This allows the phases of the third entering light and the third reflected light to be aligned and emission from the third light-emitting layer 124 to be increased. Thus, the luminance of the light-emitting element according to this embodiment is higher than the luminance of a light-emitting element in which the optical path length is not adjusted, in the case where the same current is applied to these light-emitting elements.

The optical path length between the electrode 102 having a reflective property and the third light-emitting layer 124 (the optical path length of the third reflected light) can be adjusted by adjusting the thickness of the second charge generation layer 114. Note that the first charge generation layer 108 and the second charge generation layer 114 can have a structure similar to that of the charge generation layer 107 described above.

The third EL layer 118 may include at least the third light-emitting layer 124. The description of the first EL layer 106 or the second EL layer 110 may be referred to for the specific structure of the third EL layer 118. Note that a light-emitting material different from the light-emitting material of the first light-emitting layer 120 and the light-emitting material of the second light-emitting layer 122 is preferably used as a light-emitting material for the third light-emitting layer 124.

Note that the structure of the light-emitting element according to this embodiment is not limited to the structures illustrated in FIGS. 1A and 1B. For example, a light-emitting element may include four or more EL layers as illustrated in FIG. 2. Each EL layer includes at least a light-emitting layer. Moreover, it is preferable that a charge generation layer be provided between one EL layer and another EL layer.

In the structure of the light-emitting element illustrated in FIG. 2, the optical path length between the electrode 102 having a reflective property and a light-emitting layer (the optical path length of reflected light) can be adjusted by adjusting the thickness of a charge generation layer.

In FIG. 2, for example, an n-th EL layer 218 (n is an integer of 4 or more) includes an m-th light-emitting layer 224 (m is an integer of 4 or more). In addition, an (n−1)th charge generation layer 214 is provided in contact with the n-th EL layer 218. In FIG. 2, the optical path length between the electrode 102 having a reflective property and the m-th light-emitting layer 224 is set to (2m−1)/4 of the peak wavelength of the emission spectrum of the m-th light-emitting layer 224. This allows the phases of m-th entering light and m-th reflected light to be aligned and emission from the m-th light-emitting layer 224 to be increased. Thus, the luminance of the light-emitting element according to this embodiment is higher than the luminance of a light-emitting element in which the optical path length is not adjusted, in the case where the same current is applied to these light-emitting elements.

FIGS. 3A to 3C each illustrate one embodiment of an EL display device in which a light-emitting element according to this embodiment. FIG. 3A is a plan view of a structure of an electrode of a display portion in the display device. Note that some components (e.g., an EL layer) are illustrated in a simple way in FIGS. 3A to 3C for easy understanding. FIG. 3B is a cross-sectional view illustrating an example of a structure of the display portion. FIG. 3C is a cross-sectional view illustrating another example of a structure of the display portion.

The display devices in FIGS. 3A to 3C are passive matrix display devices. In each of the display devices, the electrodes 102 having reflective properties processed in stripes (a first electrode 102 a having a reflective property, a second electrode 102 b having a reflective property, and a third electrode 102 c having a reflective property) and the electrodes 112 having light-transmitting properties processed in stripes (a first electrode 112 a having a light-transmitting property, a second electrode 112 b having a light-transmitting property, and a third electrode 112 c having a light-transmitting property) are stacked to form a lattice.

Layers (e.g., a first EL layer, a charge generation layer, and a second EL layer) provided between the electrode 102 having a reflective property and the electrode 112 having a light-transmitting property are each provided as a continuous film over an entire area between the electrode 102 having a reflective property and the electrode 112 having a light-transmitting property. Thus, selective deposition using a metal mask is not needed in a manufacturing process, which makes it possible to perform film formation over a large area at one time, to increase the size of the display device, and to improve the productivity. Moreover, the area of a display region in a display portion can be increased. Furthermore, defects due to the entry of particles, or the like which occurs when a metal mask is used can be prevented; thus, the display device can be manufactured with high yields.

The display device illustrated in FIG. 3B includes a first pixel 130 a and a second pixel 130 b. The first pixel 130 a includes a first light-emitting element 132 a provided over a substrate 100 and a first color filter layer 134 a provided for a counter substrate 128 in a region overlapping with the first light-emitting element 132 a. The second pixel 130 b includes a second light-emitting element 132 b provided over the substrate 100 and a second color filter layer 134 b provided for the counter substrate 128 in a region overlapping with the second light-emitting element 132 b.

The first light-emitting element 132 a includes a first conductive layer 104 a having a light-transmitting property, the first EL layer 106, the charge generation layer 107, the second EL layer 110, and the electrode 112 having a light-transmitting property which are stacked in this order over the first electrode 102 a having a reflective property. The second light-emitting element 132 b includes a second conductive layer 104 b having a light-transmitting property, the first EL layer 106, the charge generation layer 107, the second EL layer 110, and the electrode 112 having a light-transmitting property which are stacked in this order over the second electrode 102 b having a reflective property. The light-emitting element illustrated in FIG. 1A can be used as the first light-emitting element 132 a and the second light-emitting element 132 b.

Note that the first electrode 102 a having a reflective property, the second electrode 102 b having a reflective property, the first conductive layer 104 a having a light-transmitting property, and the second conductive layer 104 b having a light-transmitting property can be processed into a desired shape in a photolithography step and an etching step. Thus, a fine pattern can be formed with good controllability, which makes it possible to obtain a high-definition display device.

Further, when the first conductive layer 104 a having a light-transmitting property and the second conductive layer 104 b having a light-transmitting property are provided independently in each pixel, crosstalk can be prevented even in the case where the thickness of the conductive layer having a light-transmitting property is extremely large or the case where the conductivity of the conductive layer having a light-transmitting property is high.

An insulating layer 126 having openings is formed over the first conductive layer 104 a having a light-transmitting property and the second conductive layer 104 b having a light-transmitting property. The first EL layer 106 is in contact with the first conductive layer 104 a having a light-transmitting property and the second conductive layer 104 b having a light-transmitting property through the openings. The insulating layer 126 is formed using an organic insulating material such as polyimide, acrylic, polyamide, or epoxy, or an inorganic insulating material. It is particularly preferable that the insulating layer 126 be formed using a photosensitive resin material to have an opening over each of the first conductive layer 104 a having a light-transmitting property and the second conductive layer 104 b having a light-transmitting property so that the sidewall of the opening is formed to have a tilted surface with continuous curvature. The insulating layer 126 may be tapered or inversely tapered.

Plastic (organic resin), glass, quartz, or the like can be used for the substrate 100. As an example of plastic, a member made of polycarbonate, polyarylate, polyethersulfone, or the like can be given. Plastic is preferably used for the substrate 100, in which case a reduction in the weight of the display device can be achieved. Alternatively, a sheet with a high barrier property against water vapor and a high heat radiation property (e.g., a sheet including diamond like carbon (DLC)) can be used for the substrate 100.

Although not illustrated, a structure in which an inorganic insulator is provided over the substrate 100 may be employed. The inorganic insulator functions as a protective layer or a sealing film which blocks an external contaminant such as water. By providing the inorganic insulator, deterioration of the light-emitting element can be suppressed; thus, the durability and lifetime of the display device can be improved.

A single layer or a stack of a nitride film and a nitride oxide film can be used as the inorganic insulator. Specifically, the inorganic insulator can be formed using silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, aluminum nitride, aluminum oxynitride, or the like by a CVD method, a sputtering method, or the like depending on the material. It is preferable that the inorganic insulator be funned using silicon nitride by a CVD method. The thickness of the inorganic insulator may be greater than or equal to 100 nm and less than or equal to 1 μm. Alternatively, an aluminum oxide film, a DLC film, a carbon film containing nitrogen, or a film containing zinc sulfide and silicon oxide (ZnS.SiO₂ film) may be used as the inorganic insulator.

Alternatively, a thin glass substrate can be used as the inorganic insulator. For example, a glass substrate with a thickness greater than or equal to 30 μm and less than or equal to 100 μm can be used.

A metal plate may be provided on a bottom surface of the substrate 100 (a surface opposite to the surface over which the light-emitting element is provided). In the case where an inorganic insulator is provided, a metal plate may be used instead of the substrate 100. Although there is no particular limitation on the thickness of the metal plate, a metal plate with a thickness greater than or equal to 10 μm and less than or equal to 200 μm is preferably used, in which case a reduction in the weight of the display device can be achieved. Further, although there is no particular limitation on the material of the metal plate, a metal such as aluminum, copper, or nickel, a metal alloy such as an aluminum alloy or stainless steel, or the like can be preferably used.

The metal plate and the substrate 100 can be bonded to each other with an adhesive layer. As the adhesive layer, a visible light curable adhesive, an ultraviolet curable adhesive, or a thermosetting adhesive can be used. As examples of materials of such adhesives, an epoxy resin, an acrylic resin, a silicone resin, a phenol resin, and the like can be given. A moisture-absorbing substance serving as a desiccant may be contained in the adhesive layer.

A metal plate has low permeability; thus, by providing the metal plate, the entry of moisture into the light-emitting element can be prevented. Consequently, by providing the metal plate, a highly reliable display device in which deterioration due to moisture is suppressed can be provided.

Note that an inorganic insulating film which covers the first light-emitting element 132 a and the second light-emitting element 132 b may be provided. The inorganic insulating film serves as a protective layer or a sealing film which blocks an external contaminant such as water. By providing the inorganic insulating film, the deterioration of the light-emitting element can be suppressed; thus, the durability and lifetime of the display device can be improved. A material similar to the material of the inorganic insulator described above can be used as a material of the inorganic insulating film.

A moisture-absorbing substance which serves as a desiccant may be provided between the substrate 100 and the counter substrate 128. The moisture-absorbing substance may be provided in a solid state such as powdery state or may be provided in a state of a film containing a moisture-absorbing substance over the first light-emitting element 132 a and the second light-emitting element 132 b by a film formation method such as a sputtering method.

A material similar to that of the substrate 100 can be used for the counter substrate 128. Note that the counter substrate 128 needs to have a property of transmitting light passing through at least the first color filter layer 134 a and the second color filter layer 134 b.

For example, a chromatic color light-transmitting resin can be used as the first color filter layer 134 a and the second color filter layer 134 b. As the chromatic color light-transmitting resin, a photosensitive organic resin or a non-photosensitive organic resin can be used. The photosensitive organic resin is preferably used, in which case the number of resist masks can be reduced, which results in the simplification of the process.

Chromatic colors are all colors except achromatic colors such as black, gray, and white. The color filter layer is formed using a material which transmits only light of the chromatic colors. As chromatic color, red, green, blue, or the like can be used. Alternatively, cyan, magenta, yellow, or the like may be used. “Transmitting only light of a chromatic color” means that light passing through the color filter layer has a peak at a wavelength of the light of the chromatic color.

The thickness of the color filter layer may be controlled to be optimal as appropriate in consideration of the relationship between the concentration of a coloring material to be contained and the transmittance of light. By making the color of the first color filter layer 134 a and the color of the second color filter layer 134 b different from each other, a display device capable of multicolor display can be obtained. Note that the color of light emitted from the first light-emitting layer 120 is preferably the same as the color of the first color filter layer 134 a and that the color of the light emitted from the second light-emitting layer 122 is preferably the same as the color of the second color filter layer 134 b.

In the display device described in this embodiment, the half width of a spectrum of light emitted from the first light-emitting layer 120 can be reduced by adjusting the optical path length between the first electrode 102 a having a reflective property and the first light-emitting layer 120 and by utilizing light interference. In a similar manner, the half width of a spectrum of light emitted from the second light-emitting layer 122 can be reduced by adjusting the optical path length between the second electrode 102 b having a reflective property and the second light-emitting layer 122 and by utilizing light interference. Thus, when the first color filter layer 134 a and the light emitted from the first light-emitting layer have the same color, the concentration of a coloring material of the first color filter layer 134 a can be low. In addition, the thickness of the first color filter layer 134 a can be small. The same can be applied to the second color filter layer 134 b. As a result, light absorption by the first color filter layer 134 a or the second color filter layer 134 b can be reduced; thus, the use efficiency of light can be improved.

The example in which the first color filter layer 134 a and the second color filter layer 134 b are provided on the inner side of the counter substrate 128 is described in this embodiment. However, one embodiment of the present invention is not limited thereto. The first color filter layer 134 a and the second color filter layer 134 b can be provided on the outer side of the counter substrate 128 (i.e., on the opposite side to the light-emitting elements).

Alternatively, a light-transmitting resin layer with a chromatic color which functions as a color filter layer may be formed over the first light-emitting element 132 a and the second light-emitting element 132 b.

A light-blocking layer may be provided in a region between the first color filter layer 134 a and the second color filter layer 134 b (i.e., a region overlapping with the insulating layer 126). The light-blocking layer is formed using a light-blocking material which reflects or absorbs light. For example, a black organic resin can be used, which can be formed by mixing a black resin of a pigment material, carbon black, titanium black, or the like into a resin material such as photosensitive or non-photosensitive polyimide. Alternatively, a light-blocking metal film can be used, which is made of chromium, molybdenum, nickel, titanium, cobalt, copper, tungsten, aluminum, or the like, for example.

There is no particular limitation on the formation method of the light-blocking layer, and a dry method such as an evaporation method, a sputtering method, or a CVD method, or a wet method such as a spin coating method, a dip coating method, a spray coating method, a droplet discharge method (e.g., ink jetting), a screen printing method, or an offset printing method may be used depending on the material. If needed, an etching method (dry etching or wet etching) may be employed to form a desired pattern.

The light-blocking layer can prevent light from leaking to an adjacent pixel. Therefore, by providing the light-blocking layer, an image can be displayed with high contrast and high definition.

The display device illustrated in FIG. 3C includes a first pixel 230 a, a second pixel 230 b, and a third pixel 230 c. The first pixel 230 a includes a first light-emitting element 232 a provided over the substrate 100 and the first color filter layer 134 a provided for the counter substrate 128 in a region overlapping with the first light-emitting element 232 a. A second pixel 230 b includes a second light-emitting element 232 b provided over the substrate 100 and the second color filter layer 134 b provided for the counter substrate 128 in a region overlapping with the second light-emitting element 232 b. A third pixel 230 c includes a third light-emitting element 232 c provided over the substrate 100 and a third color filter layer 134 c provided for the counter substrate 128 in a region overlapping with the third light-emitting element 232 c.

For example, the first color filter layer 134 a, the second color filter layer 134 b, the third color filter layer 134 c are made to be blue, green, and red, respectively, whereby a display device capable of full-color display can be obtained.

The first light-emitting element 232 a includes the first electrode 102 a having a reflective property, and the first conductive layer 104 a having a light-transmitting property, the first EL layer 106, the first charge generation layer 108, the second EL layer 110, the second charge generation layer 114, the third EL layer 118, and the electrode 112 having a light-transmitting property which are stacked in this order over the first electrode 102 a having a reflective property. The second light-emitting element 232 b includes the second electrode 102 b having a reflective property, and the second conductive layer 104 b having a light-transmitting property, the first EL layer 106, the first charge generation layer 108, the second EL layer 110, the second charge generation layer 114, the third EL layer 118, and the electrode 112 having a light-transmitting property which are stacked in this order over the second electrode 102 b having a reflective property. The third light-emitting element 232 c includes a third electrode 102 c having a reflective property, and the third conductive layer 104 c having a light-transmitting property, the first EL layer 106, the first charge generation layer 108, the second EL layer 110, the second charge generation layer 114, and the third EL layer 118, and the electrode 112 having a light-transmitting property which are stacked in this order over the third electrode 102 c having a reflective property. The light-emitting element illustrated in FIG. 1B can be used as the first light-emitting element 232 a, the second light-emitting element 232 b, and the third light-emitting element 232 c.

In FIG. 3C, light emitted from the first light-emitting layer and the first color filter layer 134 a preferably have the same color; light emitted from the second light-emitting layer and the second color filter layer 134 b preferably have the same color; and light emitted from the third light-emitting layer and the third color filter layer 134 c preferably have the same color. This makes it possible to reduce light absorption by the first color filter layer 134 a, the second color filter layer 134 b, and the third color filter layer 134 c; thus, use efficiency of light can be improved.

In the display device described in this embodiment, the optical path length between the electrode having a reflective property and each of the plurality of light-emitting layers in the light-emitting elements is optimized in accordance with the wavelength of light emitted from each light-emitting layer, whereby light of different colors can be extracted from pixels with high emission efficiency. Moreover, the half width of an emission spectrum of each light-emitting layer is reduced; thus, light with high color purity can be extracted from the color filter layer with high efficiency.

Furthermore, the light-emitting layer is formed as a continuous film without performing selective deposition of light-emitting layers in pixels with the use of a metal mask. This can prevent a reduction in yield or a complicated process caused by the use of a metal mask. Thus, a high-definition and low-power-consumption display device can be provided.

The structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

Embodiment 2

In this embodiment, an active matrix display device that is one embodiment of the present invention will be described with reference to FIGS. 4A and 4B. FIG. 4A is a plan view illustrating a display device. FIG. 4B is a cross-sectional view taken along line A-B and C-D in FIG. 4A.

In the display device illustrated in FIGS. 4A and 4B, an element substrate 410 and a sealing substrate 404 are attached to each other with a sealant 405, and a driver circuit portion (a source-side driver circuit 401 and a gate side driver circuit 403) and a pixel portion 402 including a plurality of pixels are provided.

Note that a wiring 408 is a wiring for transmitting signals that are to be inputted to the source side driver circuit 401 and the gate side driver circuit 403, and receives a video signal, a clock signal, a start signal, a reset signal, and the like from a flexible printed circuit (FPC) 409 which serves as an external input terminal. Although only the FPC is illustrated here, a printed wiring board (PWB) may be attached to the FPC. The display device in this specification includes not only a display device itself but also a display device to which an FPC or a PWB is attached.

The driver circuit portion (the source side driver circuit 401 and the gate side driver circuit 403) includes a plurality of transistors. A plurality of pixels included in the pixel portion 402 each include a switching transistor, a current controlling transistor, and a first electrode electrically connected to a drain electrode of the current controlling transistor.

Although the driver circuit portion (the source side driver circuit 401 and the gate side driver circuit 403) and the pixel portion 402 are formed over the element substrate 410, FIG. 4B illustrates the source side driver circuit 401 which is the driver circuit portion and three pixels in the pixel portion 402.

The plurality of pixels in the pixel portion 402 each include the switching transistor, the current controlling transistor, and the first electrode electrically connected to a drain electrode of the current controlling transistor. The plurality of pixels include at least two pixels. In this embodiment, an example is described in which pixels of three colors, a blue (B) pixel 420 a, a green (G) pixel 420 b, and a red, (R) pixel 420 c, are provided.

The pixels 420 a, 420 b, and 420 c respectively include color filter layers 434 a, 434 b, 434 c; light-emitting elements 418 a, 418 b, and 418 c; and transistors 412 a, 412 b, and 412 c which are respectively electrically connected to the light-emitting elements 418 a, 418 b, and 418 c and function as switching transistors.

The light-emitting elements 418 a, 418 b, and 418 c respectively include a stacked layer of an electrode 413 a having a reflective property and a conductive layer 415 a having a light-transmitting property, a stacked layer of an electrode 413 b having a reflective property and a conductive layer 415 b having a light-transmitting property, and a stacked layer of an electrode 413 c having a reflective property and a conductive layer 415 c having a light-transmitting property. In addition, the light-emitting elements 418 a, 418 b, and 418 c include, over the respective stacked layers, a first EL layer 431 in which a first light-emitting layer is provided, a first charge generation layer 432, a second EL layer 433 in which a second light-emitting layer is provided, a second charge generation layer 435, a third EL layer 436 in which a third light-emitting layer is provided, and an electrode 417 having a light-transmitting property.

The distance between each of the electrodes 413 a, 413 b, and 413 c having reflective properties and the first light-emitting layer is adjusted so as to be one-quarter of the peak wavelength of the emission spectrum of the first light-emitting layer; the distance between each of the electrodes 413 a, 413 b, and 413 c having reflective properties and the second light-emitting layer is adjusted by the first charge generation layer 432 so as to be three-quarters of the peak wavelength of the emission spectrum of the second light-emitting layer; and the distance between each of the electrodes 413 a, 413 b, and 413 c having reflective properties and the third light-emitting layer is adjusted by the second charge generation layer 435 so as to be five-quarters of the peak wavelength of the emission spectrum of the third light-emitting layer. The distance between the electrode having a reflective property and the first light-emitting layer can be adjusted by controlling the thickness of a thin film provided between the electrode having a reflective property and the first light-emitting layer (e.g., a conductive layer having a light-transmitting property or a functional layer included in the first EL layer). Moreover, the distance between the electrode having a reflective property and the second light-emitting layer or the third light-emitting layer can be adjusted by controlling the thickness of the first charge generation layer 432 or the second charge generation layer 435.

A color filter layer which transmits light with a peak wavelength of the emission spectrum of the first light-emitting layer is used as the color filter layer 434 a; a color filter layer which transmits light with a peak wavelength of the emission spectrum of the second light-emitting layer is used as the color filter layer 434 b; and a color filter layer which transmits light with a peak wavelength of the emission spectrum of the third light-emitting layer is used as the color filter layer 434 c.

For example, the color filter layer 434 a of the blue (B) pixel 420 a may be blue, the color filter layer 434 b of the green (G) pixel 420 b may be green, and the color filter layer 434 c of the red (R) pixel 420 c may be red.

By optimizing the distance between the electrode having a reflective property and each of the first light-emitting layer, the second light-emitting layer, and the third light-emitting layer in accordance with the wavelength of color of light emitted from each light-emitting layer, light of different colors can be extracted from the pixels with high emission efficiency. Moreover, the half width of an emission spectrum of each light-emitting layer is reduced; thus, light with high color purity can be extracted from each of the color filter layers 434 a, 434 b, and 434 c with high efficiency.

For example, the first light-emitting layer is made to emit red light, the second light-emitting layer is made to emit green light, and the third light-emitting layer is made to emit blue light, whereby white light can be extracted from the display device without providing a color filter layer. Thus, a display device including pixels corresponding to R, B, and W can be manufactured. A light-emitting device which emits white light can also be manufactured. Moreover, the light-emitting device can be used as a lighting device.

Furthermore, the light-emitting layer is formed as a continuous film without performing selective deposition of light-emitting layers in pixels with the use of a metal mask. This can prevent a reduction in yield or a complicated process caused by the use of a metal mask. Thus, a high-definition display device with high color reproducibility can be provided. In addition, a low-power-consumption display device can be provided.

A CMOS circuit, which is a combination of an n-channel transistor 423 and a p-channel transistor 424, is formed for the source side driver circuit 401. The driver circuit may be any of a variety of circuits formed with transistors, such as a CMOS circuit, a PMOS circuit, or an NMOS circuit. Although the example in which the source side driver circuit and the gate side driver circuit are formed over a substrate is described in this embodiment, one embodiment of the present invention is not limited thereto. All or part of the source side driver circuit and the gate side driver circuit may be formed outside a substrate, not over the substrate.

Note that an insulator 414 is formed to cover end portions of the electrodes 413 a, 413 b, and 413 c having reflective properties and end portions of the conductive layers 415 a, 415 b, and 415 c having light-transmitting properties. Here, the insulator 414 is formed using a positive type photosensitive acrylic resin film.

In order to improve the coverage, the insulator 414 is provided such that either an upper end portion or a lower end portion of the insulator 414 has a curved surface with a curvature. For example, when positive photosensitive acrylic is used as a material for the insulator 414, it is preferable that only an upper end portion of the insulator 414 have a curved surface with a radius of curvature (0.2 μm to 3 μm). For the insulator 414, it is also possible to use either a negative type that becomes insoluble in an etchant by light irradiation or a positive type that becomes soluble in an etchant by light irradiation.

Any of the materials described in Embodiment 1 can be used for each of the color filter layers 434 a, 434 b, and 434 c, the electrodes 413 a, 413 b, and 413 c having reflective properties, the conductive layers 415 a, 415 b, and 415 c having light-transmitting properties, the first EL layer 431, the first charge generation layer 432, the second EL layer 433, the second charge generation layer 435, the third EL layer 436, and the electrode 417 having a light-transmitting property.

The sealing substrate 404 is attached to the element substrate 410 with the sealant 405; thus, a light-emitting element 418 is provided in a space 407 enclosed by the element substrate 410, the sealing substrate 404, and the sealant 405. Note that the space 407 is filled with a filler and may be filled with an inert gas (e.g., nitrogen or argon), an organic resin, or the sealant 405. A substance having a hygroscopic property may be used as the organic resin and the sealant 405.

Note that an epoxy-based resin is preferably used as the sealant 405. It is preferable that such a material allow as little moisture and oxygen as possible to penetrate. As a material for the sealing substrate 404, a glass substrate, a quartz substrate, or a plastic substrate made of fiberglass-reinforced plastics (FRP), polyvinyl fluoride (PVF), polyester, acrylic, or the like can be used.

As in this embodiment, an insulating film 411 which serves as a base film may be provided between the element substrate 410 and a semiconductor layer of the transistor. The insulating film has a function of preventing diffusion of an impurity element from the element substrate 410 and can be formed to have a single-layer structure or a stacked-layer structure using one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film.

There is no particular limitation on the structure of the transistor which can be used in the display device disclosed in this specification; for example, a staggered type transistor or a planar type transistor having a top-gate structure or a bottom-gate structure can be used. The transistor may have a single-gate structure in which one channel formation region is formed, a double-gate structure in which two channel formation regions are formed, or a triple-gate structure in which three channel formation regions are formed. Alternatively, the transistor may have a dual-gate structure including two gate electrode layers positioned over and below a channel region with a gate insulating layer therebetween.

The gate electrode layers can be formed to have a single-layer or stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material containing any of these materials as its main component.

For example, as a two-layer structure of the gate electrode layer, the following structures are preferable: a two-layer structure of an aluminum layer and a molybdenum layer stacked thereover, a two-layer structure of a copper layer and a molybdenum layer stacked thereover, a two-layer structure of a copper layer and a titanium nitride layer or a tantalum nitride layer stacked thereover, and a two-layer structure of a titanium nitride layer and a molybdenum layer. As a three-layer structure, a three-layer structure in which a tungsten layer or a tungsten nitride layer, an alloy of aluminum and silicon or an alloy of aluminum and titanium, and a titanium nitride layer or a titanium layer are stacked is preferable.

The gate insulating layer can be formed to have a single-layer structure or a stacked-layer structure using a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, and/or a silicon nitride oxide layer by a plasma CVD method, a sputtering method, or the like. Alternatively, a silicon oxide layer formed by a CVD method using an organosilane gas can be used as the gate insulating layer. As an organosilane gas, a silicon-containing compound such as tetraethoxysilane (TEOS) (chemical formula: Si(OC₂H₅)₄), tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄), tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (chemical formula: SiH(OC₂H₅)₃), or trisdimethylaminosilane (chemical formula: SiH(N(CH₃)₂)₃) can be used.

A material of the semiconductor layer is not particularly limited and may be determined as appropriate in accordance with the characteristics needed for the transistors 412 a, 412 b, 412 c, 423, and 424. Examples of materials which can be used for the semiconductor layer will be given.

As the material of the semiconductor layer, any of the following can be used: an amorphous semiconductor manufactured by or a sputtering method or a vapor-phase growth method using a semiconductor material gas typified by silane or germane; a polycrystalline semiconductor formed by crystallizing the amorphous semiconductor with the use of light energy or thermal energy; and a microcrystalline semiconductor. The semiconductor layer can be formed by a sputtering method, an LPCVD method, a plasma CVD method, or the like.

A single crystal semiconductor made of silicon or silicon carbide can be used for the semiconductor layer. When a single crystal semiconductor is used for the semiconductor layer, the size of the transistor can be reduced; thus, higher resolution pixels in a display portion can be obtained. In the case where a single crystal semiconductor is used for the semiconductor layer, an SOI substrate in which a single crystal semiconductor layer is provided can be used. Alternatively, a semiconductor substrate such as a silicon wafer may be used.

A typical example of an amorphous semiconductor is hydrogenated amorphous silicon, and a typical example of a crystalline semiconductor is polysilicon and the like. Examples of polysilicon (polycrystalline silicon) include so-called high-temperature polysilicon which contains polysilicon formed at a process temperature of 800° C. or higher as its main component, so-called low-temperature polysilicon which contains polysilicon formed at a process temperature of 600° C. or lower as its main component, and polysilicon obtained by crystallizing amorphous silicon with the use of an element that promotes crystallization. Needless to say, a microcrystalline semiconductor or a semiconductor partly containing a crystal phase can be used as described above.

Further, an oxide semiconductor may be used. As the oxide semiconductor, the following can be used: an oxide of four metal elements such as an In—Sn—Ga—Zn—O-based oxide semiconductor; an oxide of three metal elements such as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxide semiconductor; or an oxide of two metal elements such as an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxide semiconductor, or In—Ga—O-based oxide semiconductor; an In—O-based oxide semiconductor; a Sn—O-based oxide semiconductor; or a Zn—O-based oxide semiconductor. Further, SiO₂ may be contained in the above oxide semiconductor. Here, for example, an In—Ga—Zn—O-based oxide semiconductor is an oxide containing at least In, Ga, and Zn, and the composition ratio of the elements is not particularly limited. The In—Ga—Zn—O-based oxide semiconductor may contain an element other than In, Ga; and Zn.

A thin film expressed by a chemical formula of InMO₃(ZnO)_(m)(m>0) can be used for the oxide semiconductor layer. Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In the case where an In—Zn—O-based material is used as the oxide semiconductor, the atomic ratio thereof is In/Zn=0.5 to 50, preferably In/Zn=1 to 20, further preferably In/Zn=1.5 to 15. When the atomic ratio of Zn is in the above preferred range, the field-effect mobility of a transistor can be improved. Here, when the atomic ratio of the compound is In:Zn:O═X:Y:Z, the relation of Z>1.5X+Y is satisfied.

For the oxide semiconductor layer, a crystalline oxide semiconductor which has neither a single crystal structure nor an amorphous structure and has c-axis alignment (also referred to as c-axis aligned crystalline (CAAC) oxide semiconductor).

As examples of materials for a wiring layer serving as a source electrode layer or a drain electrode layer, the following are given: an element selected from Al, Cr, Ta, Ti, Mo, and W; an alloy containing any of the above elements as its component; an alloy film containing a combination of any of these elements; and the like. In the case where heat treatment is performed, a conductive film preferably has heat resistance high enough to withstand the heat treatment. Since the use of Al alone brings disadvantages such as low heat resistance and a tendency for corrosion, aluminum is used in combination with a conductive material having heat resistance. As the conductive material having heat resistance, which is combined with Al, it is possible to use an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of these elements as its component, an alloy containing a combination of any of these elements, or a nitride containing any of these elements as its component.

An inorganic insulating film or an organic insulating film formed by a dry method or a wet method can be used for an insulating film 419 which covers the transistors. For example, a silicon nitride film, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, a tantalum oxide film, or a gallium oxide film which is formed by a CVD method, a sputtering method, or, the like can be used. Alternatively, an organic material such as polyimide, acrylic, benzocyclobutene, polyamide, or an epoxy resin can be used. Other than the above organic materials, a low-dielectric constant material (a low-k material), a siloxane-based resin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), or the like can be used.

Note that the siloxane-based resin corresponds to a resin including a Si—O—Si bond formed using a siloxane-based material as a starting material. The siloxane-based resin may include, as a substituent, an organic group (e.g., an alkyl group or an aryl group) or a fluoro group. The organic group may include a fluoro group. A siloxane-based resin is applied by a coating method and baked; thus, the insulating film 419 can be formed.

Note that the insulating film 419 may be formed by stacking a plurality of insulating films formed using any of the above-described materials. For example, a structure may be employed in which an organic resin film is stacked over an inorganic insulating film.

In the above manner, the active matrix display device including the light-emitting element of one embodiment of the present invention can be obtained.

Note that this embodiment can be freely combined with any of the other embodiments.

Embodiment 3

A display device disclosed in this specification can be applied to a variety of electronic appliances (including game machines). Examples of electronic devices include a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone handset (also referred to as a mobile phone or a mobile phone device), a portable game console, a portable information terminal, an audio reproducing device, and a large-sized game machine such as a pachinko machine.

FIG. 5A illustrates a laptop personal computer, which includes a main body 3001, a housing 3002, a display portion 3003, a keyboard 3004, and the like. By applying the display device described in Embodiment 1 or 2 to the display portion 3003, the laptop personal computer can have a high level of definition and consumes a small amount of power.

FIG. 5B illustrates a personal digital assistant (PDA), which includes a display portion 3023, an external interface 3025, an operation button 3024, and the like in a main body 3021. The personal digital assistant also includes a stylus 3022 as an accessory for operation. By applying the display device described in Embodiment 1 or 2 to the display portion 3023, the personal digital assistant (PDA) can have a high level of definition and consumes a small amount of power.

FIG. 5C illustrates an e-book reader, which includes two housings, a housing 2701 and a housing 2703. The housing 2701 and the housing 2703 are combined with a hinge 2711 so that the e-book reader can be opened and closed with the hinge 2711 as an axis. Such a structure enables the e-book reader to operate like a paper book.

A display portion 2705 and a display portion 2707 are incorporated in the housing 2701 and the housing 2703, respectively. The display portion 2705 and the display portion 2707 may display one image or different images. In the case where the display portion 2705 and the display portion 2707 display different images, for example, text can be displayed on a display portion on the right side (the display portion 2705 in FIG. 5C) and graphics can be displayed on a display portion on the left side (the display portion 2707 in FIG. 5C). By applying the display device described in Embodiment 1 or 2 to the display portion 2705 and the display portion 2707, the e-book reader can have a high level of definition and consumes a small amount of power. In the case where a semi-transmissive display device or a reflective display device is used for the display portion 2705, a solar battery may be provided so that the solar battery can generate power and a battery can be charged for the use in relatively bright conditions. Note that when a lithium ion battery is used as the battery, an advantage such as reduction in size can be obtained.

Further, FIG. 5C illustrates an example in which the housing 2701 is provided with an operation portion and the like. For example, the housing 2701 is provided with a power switch 2721, operation keys 2723, a speaker 2725, and the like. Pages can be turned with the operation key 2723. Note that a keyboard, a pointing device, or the like may also be provided on the surface of the housing, on which the display portion is provided. Furthermore, an external connection teiuiinal (an earphone terminal, a USB terminal, or the like), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing. Further, the e-book reader may have a function of an electronic dictionary.

The e-book reader may wirelessly transmit and receive data. Through wireless communication, desired book data or the like can be purchased and downloaded from an electronic book server.

FIG. 5D illustrates a mobile phone, which includes two housings, a housing 2800 and a housing 2801. The housing 2801 includes a display panel 2802, a speaker 2803, a microphone 2804, a pointing device 2806, a camera lens 2807, an external connection terminal 2808, and the like. In addition, the housing. 2800 includes a solar cell 2810 having a function of charge of the mobile phone, an external memory slot 2811, and the like. Further, an antenna is incorporated in the housing 2801. By applying the display device described in Embodiment 1 or Embodiment 2 to the display panel 2802, the mobile phone can have a higher level of definition and consumes a smaller amount of power.

The display panel 2802 is provided with a touch panel. A plurality of operation keys 2805 which are displayed as images are illustrated by dashed lines in FIG. 5D. Note that a boosting circuit by which voltage output from the solar cell 2810 is increased to be sufficiently high for each circuit is also included.

The display direction in the display panel 2802 is changed as appropriate depending on a usage pattern. Further, the mobile phone is provided with the camera lens 2807 on the same surface as the display panel 2802; thus, it can be used as a video phone. The speaker 2803 and the microphone 2804 can be used for videophone calls, recording and playing sound, and the like without limitation to voice calls. Moreover, the housing 2800 and the housing 2801 developed as illustrated in FIG. 5D can be slid so that one is lapped over the other; thus, the size of the mobile phone can be reduced, which makes the mobile phone suitable for being carried.

The external connection terminal 2808 can be connected to an AC adapter and various types of cables such as a USB cable, and charging and data communication with a personal computer are possible. Moreover, a large amount of data can be stored by inserting a storage medium into the external memory slot 2811 and can be moved.

Further, an infrared communication function, a television reception function, or the like may be provided in addition to the above functions.

FIG. 5E illustrates a digital video camera, which includes a main body 3051, a display portion A 3057, an eyepiece 3053, an operation switch 3054, a display portion B 3055, a battery 3056, and the like. By applying the display device described in Embodiment 1 or Embodiment 2 to each of the display portion A 3057 and the display portion B 3055, the digital video camera can have a higher level of definition and consumes a smaller amount of power.

FIG. 5F illustrates a television device in which a display portion 9603 and the like are incorporated in a housing 9601. Images can be displayed on the display portion 9603. Here, the housing 9601 is supported by a stand 9605. By applying the display device described in Embodiment 1 or Embodiment 2 to the display portion 9603, the television device can have a higher level of definition and consumes a smaller amount of power.

The television device can be operated by an operation switch of the housing 9601 or a separate remote controller. Further, the remote controller may be provided with a display portion for displaying data output from the remote controller.

Note that the television device is provided with a receiver, a modem, and the like. With the use of the receiver, general television broadcasting can be received. Moreover, when the display device is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers) information communication can be performed.

This embodiment can be implemented in an appropriate combination with any of the structures described in the other embodiments.

Note that the structure described in this embodiment can be combined with the structure described in Embodiment 1 or 2 as appropriate.

Example

In this example, measurement results of characteristics of a light-emitting element according to one embodiment of the present invention will be described with reference to a drawing and tables.

In this example, a manufacturing method of the light-emitting element will be described with reference to FIG. 6.

Shown below are structural formulae of organic compounds used in this example (BPhen, 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: PCzPA), 9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene (abbreviation: CzPA), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine (abbreviation: 1,6 mMemFLPAPrn), 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), and (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-Me)₂(acac)).

As an electrode 1101 having a reflective property of the light-emitting element, an aluminum-nickel-lanthanum alloy film was formed over a substrate 1100 which was a glass substrate by a sputtering method. In this example, the electrode 1101 having a reflective property was used as an anode. Note that the electrode area was 2 mm×2 mm.

Next, indium tin oxide containing silicon oxide (ITSO) was deposited over the electrode 1101 having a reflective property by a sputtering method to form a conductive layer 1104 having a light-transmitting property with a thickness of 10 nm.

Next, the substrate 1100 provided with the electrode 1101 having a reflective property and the conductive layer 1104 having a light-transmitting property was fixed to a substrate holder provided in a vacuum evaporation apparatus such that the surface on which the electrode 1101 having a reflective property and the conductive layer 1104 having a light-transmitting property were formed faced downward, and then the pressure was reduced to about 10⁻⁴ Pa. After that, PCzPA and molybdenum(VI) oxide were co-evaporated on the conductive layer 1104 having a light-transmitting property to form a hole-injection layer 1111. The weight ratio of PCzPA to molybdenum oxide was adjusted to be 1:0.5 (=PCzPA: molybdenum oxide). The thickness of the hole-injection layer 1111 was 10 nm. Note that the co-evaporation method refers to an evaporation method in which evaporation is carried out from a plurality of evaporation sources at the same time in one treatment chamber.

Next, PCzPA was deposited to a thickness of 25 nm on the hole-injection layer 1111 to form a hole-transport layer 1112.

On the hole-transport layer 1112, CzPA and 1,6mMemFLPAPrn were co-evaporated so that the weight ratio of CzPA to 1,6mMemFLPAPrn was 1:0.05 (=CzPA: 1,6mMemFLPAPrn) to form a light-emitting layer 1113. The thickness of the light-emitting layer 1113 was 30 nm.

On the light-emitting layer 1113, CzPA was deposited to a thickness of 5 nm to form an electron-transport layer 1114 a.

On the electron-transport layer 1114 a, bathophenanthroline (abbreviation: BPhen) was deposited to a thickness of 15 nm to form an electron-transport layer 1114 b.

On the electron-transport layer 1114 b, lithium oxide (Li₂O) was evaporated to a thickness of 0.1 nm to form an electron-injection layer 1115 a, and on the electron-injection layer 1115 a, copper(II) phthalocyanine (abbreviation: CuPc) was evaporated to a thickness of 2 nm to form an electron-injection layer 1115 b.

On the electron-injection layer 1115 b, PCzPA and molybdenum(VI) oxide were co-evaporated to form a charge generation layer 1102. The weight ratio of PCzPA to molybdenum oxide was adjusted to be 1:0.5 (=PCzPA: molybdenum oxide). The thickness of the charge generation layer 1102 was 120 nm.

On the charge generation layer 1102, BPAFLP was deposited to a thickness of 20 nm to form a hole-transport layer 1212.

On the hole-transport layer 1212, 2mDBTPDBq-II, PCBA1BP, and Ir(mppr-Me)₂(acac) were co-evaporated so that the weight ratio of 2mDBTPDBq-II to PCBA1BP and Ir(mppr-Me)₂(acac) was 0.6:0.2:0.06 (=2mDBTPDBq-II: PCBA1BP: Ir(mppr-Me)₂(acac)) to form a light-emitting layer 1213. The thickness of the light-emitting layer 1213 was 15 nm.

On the light-emitting layer 1213, 2mDBTPDBq-II and Ir(mppr-Me)₂(acac) were co-evaporated so that the weight ratio of 2mDBTPDBq-II to Ir(mppr-Me)₂(acac) was 1:0.06 (=2mDBTPDBq-II: Ir(mppr-Me)₂(acac)) to form a light-emitting layer 1313. The thickness of the light-emitting layer 1313 was 15 nm.

On the light-emitting layer 1313, 2mDBTPDBq-II was deposited to a thickness of 20 nm to form an electron-transport layer 1214 a.

On the electron-transport layer 1214 a, BPhen was deposited to a thickness of 10 nm to form an electron-transport layer 1214 b.

On the electron-transport layer 1214 b, lithium oxide (Li₂O) was evaporated to a thickness of 0.1 nm to form an electron-injection layer 1215 a, and on the electron-injection layer 1215 a, copper(II) phthalocyanine (abbreviation: CuPc) was evaporated to a thickness of 2 nm to form an electron-injection layer 1215 b.

On the electron-injection layer 1215 b, BPAFLP and molybdenum(VI) oxide were co-evaporated to form a charge generation layer 1105. The weight ratio of BPAFLP to molybdenum oxide was adjusted to be 1:0.5 (=BPAFLP: molybdenum oxide). The thickness of the charge generation layer 1105 was 20 nm.

On the charge generation layer 1105, indium tin oxide (ITO) was deposited to a thickness of 110 nm by a sputtering method to form an electrode 1103 having a light-transmitting property.

Through the above steps, the light-emitting element of this example was manufactured.

Note that in all of the above evaporation steps, a resistance heating method was employed.

Table 1 shows the element structure of the light-emitting element of this example, which was manufactured in the above manner.

TABLE 1 1101 1104 1111 1112 1113 1114a light- Al—Ni—La ITSO PCzPA:MoOx PCzPA CzPA:1,6- CzPA emitting 10 nm (=1:0.5) 25 nm mMemFLPA 5 nm element 10 nm Prn (=1:0.05) 30 nm 1114b 1115a 1115b 1102 1212 1213 BPhen Li₂O CuPc PCzPA:MoOx BPAFLP 2mDBTPDBq- 15 nm 0.1 nm 2 nm (=1:0.5) 20 nm II:PCBA1BP:Ir(mppr- 120 nm Me)₂acac (=0.6:0.2:0.06) 15 nm 1313 1214a 1214b 1215a 1215b 1105 1103 2mDBTPDBq-II:Ir(mppr- 2mDBTPD BPhen Li₂O CuPc BPAFLP:MoOx ITO Me)₂acac Bq-II 10 nm 0.1 nm 2 nm (=1:0.5) 110 nm (=1:0.06) 20 nm 20 nm 15 nm

The light-emitting element was sealed with a glass substrate in a glove box under a nitrogen atmosphere so as not to be exposed to the air.

FIG. 7 shows the emission spectrum of the light-emitting element. In FIG. 7, the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). As shown in FIG. 7, the emission spectrum of the light-emitting element has a peak around 465 nm, which is attributed to the light-emitting layer 1113, and a peak around 586 nm, which is attributed to the light-emitting layer 1213. A wavelength of 465 nm was obtained as the peak of the emission spectrum of the light-emitting layer 1113. A wavelength of 586 nm was obtained as the peak of the emission spectrum of the light-emitting layer 1213.

The optical path length between the electrode 1101 having a reflective property and the light-emitting layer 1113 in the light-emitting element of this example is one-quarter of the peak wavelength (465 nm) of the emission spectrum of the light-emitting layer 1113. The optical path length between the electrode 1101 having a reflective property and the light-emitting layer 1213 in the light-emitting element of this example is three-quarters of the peak wavelength (586 nm) of the emission spectrum of the light-emitting layer 1213.

The optical path length is calculated by the following formula: refractive index×length (thickness). Table 2 shows the thickness and refractive index of each layer, which were used for calculating the optical path length of the light-emitting element; one-quarter of the peak wavelength (465 nm) of the emission spectrum of the light-emitting layer 1113; the refractive index of the light-emitting layer 1113; and the calculated optical path lengths.

TABLE 2 Thickness Refractive index at Optical path (nm) around 465 nm length (nm) ITSO 10 2.18 21.8 PCzPA-OMOx 10 1.91 19.1 PCzPA 25 1.92 48 Total 88.9 ¼ of peak wavelength (465 nm) of emission 116.25 spectrum of light-emitting layer 1113 Refractive index of 1.86 light-emitting layer 1113 (CzPA) Light-emitting region (distance from interface 14.7 between hole-transport layer 1112 and light- emitting layer 1113 to light-emitting region)

Table 2 shows that the optical path length between the electrode 1101 having a reflective property and a light-emitting region of the light-emitting layer 1113, which is located at about 14.7 nm from an interface with the hole-transport layer 1112, corresponds to one-quarter of the peak wavelength (465 nm) of the emission spectrum of the light-emitting layer 1113.

Table 3 shows the thickness and refractive index of each layer, which were used for calculating the optical path length of the light-emitting element; three-quarters of the peak wavelength (586 nm) of the emission spectrum of the light-emitting layer 1213; the refractive index of the light-emitting layer 1213; and the calculated optical path lengths.

TABLE 3 Thickness Refractive index at Optical path (nm) around 586 nm length (nm) ITSO 10 2.18 21.8 PCzPA-OMOx 10 1.91 19.1 PCzPA 25 1.92 48 Light-emitting 30 1.78 53.4 layer 1113 (CzPA) CzPA 5 1.78 8.9 BPhen 15 1.7 25.5 CuPc 2 1.45 2.9 PCzPA-OMOx 120 1.83 219.6 BPAFLP 20 1.74 34.8 Total 430.4 ¾ of peak wavelength (586 nm) of emission 439.5 spectrum of light-emitting layer 1213 Refractive index of 1.78 light-emitting layer 1213 (2mDBTPDBq-II) Light-emitting region (distance from interface 5.11 between hole-transport layer 1212 and light- emitting layer 1213 to light-emitting region)

Table 3 shows that the optical path length between the electrode 1101 having a reflective property and a light-emitting region of the light-emitting layer 1213, which is located at about 5.11 nm from an interface with the hole-transport layer 1212, corresponds to three-quarters of the peak wavelength (586 nm) of the emission spectrum of the light-emitting layer 1213.

At a luminance of 914 cd/m², the CIE chromaticity coordinates of the light-emitting element were (x=0.46, y=0.37), which indicates favorable white light emission. Further, at a luminance of 913 cd/m², the current efficiency was 68.0 cd/A, the external quantum efficiency was 26.8%, the voltage was 6.2 V, the current density was 1.3 mA/cm², and the power efficiency was 34.4 lm/W.

The light-emitting element of this example has favorable light distribution characteristics in accordance with the Lambertian radiation pattern. This confirmed that the light-emitting element of this example is useful as a planar light source.

This application is based on Japanese Patent Application serial no. 2011-027960 filed with the Japan Patent Office on Feb. 11, 2011, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A light-emitting device comprising at least three light-emitting elements, each of the three light-emitting elements comprising: an electrode having a reflective property; a stacked structure over the electrode having a reflective property, the stacked structure comprising: a first light-emitting layer over the electrode having a reflective property; a first charge generation layer over the first light-emitting layer; and a second light-emitting layer over the first charge generation layer, the second light-emitting layer comprising a first layer and a second layer; and an electrode having a light-transmitting property over the second light-emitting layer, wherein an optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of a peak wavelength of an emission spectrum of the first light-emitting layer, wherein an optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of a peak wavelength of an emission spectrum of the second light-emitting layer, wherein the first layer comprising a first compound, a second compound, and a phosphorescent compound, wherein the second layer comprising the first compound and the phosphorescent compound, wherein the three light-emitting elements are adjacent to each other, and wherein the stacked structure is continued from one of the three light-emitting elements to the adjacent light-emitting element.
 2. The light-emitting device according to claim 1, wherein colors of light emitted from the first light-emitting layer and the second light-emitting layer are different.
 3. The light-emitting device according to claim 1, wherein the light-emitting element emits white light.
 4. The light-emitting device according to claim 1, further comprising: a first substrate provided with the electrode having a reflective property; and a second substrate over the electrode having a light-transmitting property, wherein the second substrate is a light-transmitting substrate.
 5. The light-emitting device according to claim 1, further comprising: a second charge generation layer over the second light-emitting layer; and a third light-emitting layer between the second charge generation layer and the electrode having a light-transmitting property, wherein an optical path length between the electrode having a reflective property and the third light-emitting layer is five-quarters of a peak wavelength of an emission spectrum of the third light-emitting layer.
 6. The light-emitting device according to claim 5, wherein colors of light emitted from the first light-emitting layer, the second light-emitting layer, and the third light-emitting layer are different.
 7. The light-emitting device according to claim 1, further comprising: a hole-injection layer between the electrode having a reflective property and the first light-emitting layer; and a second charge generation layer between the second light-emitting layer and the electrode having a light-transmitting property, wherein the hole-injection layer comprises a first organic compound and an inorganic compound, wherein the first charge generation layer comprises the first organic compound and the inorganic compound, and wherein the second charge generation layer comprises a second organic compound and the inorganic compound.
 8. The light-emitting device according to claim 1, wherein the light-emitting device is a display device.
 9. A lighting device comprising the three light-emitting elements according to claim
 1. 10. A light-emitting device comprising: a first light-emitting element, the first light-emitting element comprising: an electrode having a reflective property; a stacked structure over the electrode having a reflective property, the stacked structure comprising: a first light-emitting layer over the electrode having a reflective property; a first charge generation layer over the first light-emitting layer; and a second light-emitting layer over the first charge generation layer, the second light-emitting layer comprising a first layer and a second layer; and an electrode having a light-transmitting property over the second light-emitting layer; a first color filter layer over the first light-emitting element; a second light-emitting element adjacent to the first light-emitting element; and a second color filter layer over the second light-emitting element, wherein an optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of a peak wavelength of an emission spectrum of the first light-emitting layer, wherein an optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of a peak wavelength of an emission spectrum of the second light-emitting layer, wherein the first color filter layer transmits light with a peak wavelength of the emission spectrum of the first light-emitting layer, wherein the second color filter layer transmits light with a peak wavelength of the emission spectrum of the second light-emitting layer, wherein the first layer comprising a first compound, a second compound, and a phosphorescent compound, wherein the second layer comprising the first compound and the phosphorescent compound, and wherein the stacked structure is continued from the first light-emitting element to the second light-emitting element.
 11. The light-emitting device according to claim 10, wherein colors of light emitted from the first light-emitting layer and the second light-emitting layer are different.
 12. The light-emitting device according to claim 10, further comprising: a first substrate provided with the first light-emitting element and the second light-emitting element; and a second substrate provided with the first color filter layer and the second color filter layer, wherein the second substrate is a light-transmitting substrate.
 13. The light-emitting device according to claim 10, further comprising: a hole-injection layer between the electrode having a reflective property and the first light-emitting layer; and a second charge generation layer between the second light-emitting layer and the electrode having a light-transmitting property, wherein the hole-injection layer comprises a first organic compound and an inorganic compound, wherein the first charge generation layer comprises the first organic compound and the inorganic compound, and wherein the second charge generation layer comprises a second organic compound and the inorganic compound.
 14. The light-emitting device according to claim 10, wherein the light-emitting device is a display device.
 15. A light-emitting device comprising: a plurality of light-emitting elements, the plurality of light-emitting elements each comprising: an electrode having a reflective property; a stacked structure over the electrode having a reflective property, the stacked structure comprising: a first light-emitting layer over the electrode having a reflective property; a first charge generation layer over the first light-emitting layer; a second light-emitting layer over the first charge generation layer, the second light-emitting layer comprising a first layer and a second layer; a second charge generation layer over the second light-emitting layer; and a third light-emitting layer over the second charge generation layer; and an electrode having a light-transmitting property over the third light-emitting layer; and a plurality of color filter layers provided over the plurality of light-emitting elements, wherein an optical path length between the electrode having a reflective property and the first light-emitting layer is one-quarter of a peak wavelength of an emission spectrum of the first light-emitting layer, wherein an optical path length between the electrode having a reflective property and the second light-emitting layer is three-quarters of a peak wavelength of an emission spectrum of the second light-emitting layer, wherein an optical path length between the electrode having a reflective property and the third light-emitting layer is five-quarters of a peak wavelength of an emission spectrum of the third light-emitting layer, and wherein the plurality of color filter layers comprise at least: a first color filter layer transmitting light with a peak wavelength of the emission spectrum of the first light-emitting layer; a second color filter layer transmitting light with a peak wavelength of the emission spectrum of the second light-emitting layer; a third color filter layer transmitting light with a peak wavelength of the emission spectrum of the third light-emitting layer, wherein the first layer comprising a first compound, a second compound, and a phosphorescent compound, wherein the second layer comprising the first compound and the phosphorescent compound, and wherein the stacked structure is continued from one of the plurality of light-emitting elements to the other of the plurality of light-emitting elements.
 16. The light-emitting device according to claim 15, wherein colors of light emitted from the first light-emitting layer, the second light-emitting layer, and the third light-emitting layer are different.
 17. The light-emitting device according to claim 15, further comprising: a first substrate provided with the plurality of light-emitting elements; and a second substrate provided with the plurality of color filter layers, wherein the second substrate is a light-transmitting substrate.
 18. The light-emitting device according to claim 15, wherein the light-emitting device is a display device.
 19. The light-emitting device according to claim 5, wherein white light is extracted from the light-emitting device without providing a color filter layer. 